20-03-2014, 12:13 PM
Electrical measurement of non-electrical quantities
Introduction to sensor technology
The term sensor technology is generally applied to the measurement and conver-
sion of measured values, often into coded signals. The detection of non-electrical
and electrical quantities calls for “pick-ups” – also referred to as “sensors”. The term
“sensor” means a hardware component capable of picking up a measured quanti-
ty at its input side. At one time the term sensor meant only the part of the pick-up
which directly detected a physical quantity, i.e. the measuring sensor. In the course
of increasing integration, the demarcation between measuring sensor and meas-
ured-value processing has become more and more difficult to establish, so that in
its present sense a sensor is the complete measuring device, consisting of meas-
uring sensor, interface electronics and a – not necessarily required – display. Sen-
sors are sensitive to different physical quantities. These physical quantities and their
time-related variations are converted by the sensors into electrical quantities.
In most cases, the output quantities of the sensors are electrical voltages or cur-
rents. Quantities such as frequency or resistance can also be output quantities,
however. The sensor principle is shown in Fig. 2.1.
Ideal sensors
The energy delivered to a sensor from outside, which may be of a mechanical, elec-
trical, chemical or thermal nature, is converted into electrical energy. An “ideal” sen-
sor performs the conversion without losses.
An ideal sensor for measuring pressure must not allow itself to be deformed or dis-
torted and must for this reason exhibit an infinitely high rigidity, while a sensor for
the measurement of vibration must have an infinitely high elasticity in order to oper-
ate loss-free. Since this is always the aim when developing sensors, almost every
suitable physical principle has been adopted for sensors. Special tasks can be met
by design measures, so a wide variety of sensors are available.
Active and passive sensors
Sensors can be divided roughly into active and passive sensors. If the quantity
being measured is converted directly into an electrical quantity, without auxiliary
power having to be supplied, “active” sensors are involved. The most important
representatives of this category are thermocouples, electrodynamic systems (gen-
erators), photo-elements and piezo-electric crystals.
Measurement of temperature
Temperature is a thermodynamic state-variable characterizing the heat-state of a
substance. Reproducible processes are adopted in order to define its value. Thus
the freezing point and boiling point of water are used as reference points for the
definition of the “°C” temperature scale (centigrade or Celsius scale).
The temperature-dependent properties of substances are used for temperature
measurement. Examples of this include thermal expansion in the case of expan-
sion/bimetallic thermometers, the temperature-dependence of an electrical resis-
tor in an electrical thermometer, and radiation in a radiation pyrometer.
Of all the various possibilities for converting a temperature into an electrical quan-
tity, resistance-measuring sensors with metals and semiconductor materials as a
resistor, and thermocouples are of particular importance.
Temperature indication takes place through electrical devices, whose construction
matches the sensor concerned. The scale is calibrated in units of temperature. In
order to obtain a temperature measurement with the shortest possible time-lag, the
sensors are designed in such a way that they quickly pick up the temperature of
the measuring point or environment. Also, the reading must react relatively quick-
ly when turned to the sensor, but it must still be possible to take a reading. Fig. 2.9
shows temperature measuring devices with measuring sensors for various appli-
cations.
Thermocouples
According to DIN 16 160, thermocouples are electrical thermometers whose tem-
perature sensor is formed by a thermocouple which provides a thermo-electric
e.m.f. influenced by temperature.
Electrical conductors consisting of different materials, connected at a point, form
a thermocouple. The individual conductors are referred to as “thermocouple wires”.
Two such wires consisting of different materials and electrically connected to each
other at one point are called a “thermocouple”.
Thermocouples are based on the Seebeck effect which states that, if two different
electrical conductors are joined together at one end (the measuring point) by weld-
ing or soldering, and if this junction is heated, then an electric voltage (the thermo-
electric e.m.f.) can be measured at the two free ends (comparison/reference point).
The thermo-electric e.m.f. is 0 V when the measuring point and reference point
exhibit the same temperature.
Measuring methods
There are various common methods for measuring temperature with thermocouples:
– direct-reading measurements;
– measurements using measuring amplifiers;
– measurements with compensation.
Direct indication using moving-coil measuring instruments, with a measuring range
to match the thermocouple being used, is common in practice. Measurement using
thermocouples calls for great accuracy, but also for a high internal resistance, in
order to load the thermocouple only lightly. It is not therefore possible to connect
a number of measuring instruments in parallel to a thermocouple.
Measuring amplifiers relieve measurement using thermocouples of dependence on
long line-lengths and on the measuring instrument used. What is more, the meas-
ured value is prepared by the measuring amplifier in such a way that it corresponds
to standard signals.
Problem-free further processing in telecontrol installations is thereby ensured. Fig.
2.23 shows basic circuits for measurements via measuring amplifiers.
Compensation circuits are usually incorporated in measuring amplifiers. They also
often include circuitry to linearise the characteristic.
Types of NTC thermistor
The various types of NTC thermistor are classified into particular groups, accord-
ing to their main application, from which they also derive their designation. An NTC
thermistor may, however, be used for an application different from that particular-
ly suggested. It is, for instance, quite possible for a delaying/restraining NTC ther-
mistor to be used for temperature measurement, but greater tolerances of the indi-
vidual electrical and thermal characteristic-values will have to be expected.
Because of the large number of NTC thermistors commercially available from a wide
variety of manufacturers, we have listed here only a few of Siemens’ compensa-
tion and measuring NTC thermistors (Fig. 2.30).